In a digital transmission network, data from a large number of users are serially transmitted from one network node to another network node, up to their respective final destinations.
Due to the evolution of networks towards more and more complex mixtures of sub-networks with heterogenous architectures, it is clear that there will be a future requirement to support distributed computing applications across high speed backbones that may be carrying LAN traffic, voice, video and traffic among channel-attached hosts and work stations. Perhaps the fundamental challenge for high speed networking is to minimize the processing time within each node in the network.
Packet switching is now commonly used to accommodate the bursty, multiprocess communication found in distributed computing environments. To accomplish this, packets carrying bursty data traffic can be assigned a non-real-time priority, while packets carrying voice and video traffic can be assigned a higher, real-time priority. A node in a fast packet switching network contains buffers for holding packets waiting for transmission on its communication links. Packets waiting for transmission can be held in buffers managed differently, depending on the priority, assigned to the packets.
European patent application (582537) discloses a communication network having communication nodes which can adopt a number of different service policies in order to transmit packets from different priority buffers, for instance, priority with no preemption, preemption with retransmission, and priority with resume. When no preemption is used, the packet priority is only examined to determine from which buffer to select the next packet for transmission. If a high-priority packet is placed in the buffer while a low-priority packet is being transmitted, the high-priority packet must wait until the current transmission is completed. A preemption with retransmission service policy means that the node will abort the transmission of a low-priority packet upon the arrival of a high priority packet and transmit the high-priority packet. Once all high-priority packets have been transmitted, transmission of the preempted low-priority packet will be restarted from the beginning of the packet. A preemption with resume service policy is similar except the preempted low-priority packet is restarted from the point of interruption rather than the beginning. The selection of the appropriate service policy is dependent on the characteristics of the communication link, the delay requirements of the high-priority packets, and the size of the low-priority packets.
The typical scheme used for transmitting packetized information over low speed communication links (T1 at 1.544 megabits per second) is based on the HDLC MAC-layer protocol, described, for example, in H. NUSSBAUMER: TELEINFORMATIQUE I, Presses Polytechniques Romandes, 1987, pages 301-313.
Both the priority with no preemption and the preemption with retransmission service policies can be implemented using the existing HDLC MAC-layer protocol. For preemption with resume service policy, a modified HDLC MAC-layer protocol is described wherein three types of flags are used to delimit packets for allowing high-priority packets to temporarily preempt low-priority packets. The HDLC starting, ending or idle flag is defined as the 8-bit sequence B`01111110`(X`7E`). The start preempt flag is defined as the 9-bit sequence B`011111110`and the end-preempt flag is defined as the 10-bit sequence B`0111111110`. All flags are on byte boundaries with respect to the packet data that they delineate.
This definition requires that the hardware is capable of scanning the incoming bit stream, of recognizing special non-standard flags in addition to the HDLC flags, and of running a protocol to verify a set of rules upon detection of these flags. Clearly, special hardware is necessary for that purpose.